1. Field of the Invention
The invention relates to a method for the stabilization of compositions containing peroxidase.
2. Description of the Prior Art
It is known that peroxidases, whether or not coupled to another component, are not very stable, particularly in low concentrations, and that their keeping qualities are therefore poor.
It has furthermore been shown that compositions containing peroxidases are very sensitive and vulnerable to freeze-drying.
Peroxidases are enzymes which catalyse the oxidation of certain compounds, during which oxidation a peroxide, and in particular hydrogen peroxide, functions as donor. They may be obtained from plants, for example horse radish peroxidase, from vertebrate animals, for example tryptophan pyrrolase, and from micro-organisms, such as cytochrome c peroxidase from Pseudomonas.
Peroxidases are used for very diverse purposes, including their use as reagents in diagnostic determinations, e.g. as a reagent in the glucose oxidase method for determining glucose, and their importance has increased even more since the development of the so-called enzyme immunotest, an immunological method of estimation for the demonstration of haptens, antigens and antibodies in which an immunological component, for example an antigen, coupled to an enzyme is used as a reagent. After the reaction between the component to be estimated and the added component(s), the enzyme activity of the final reaction mixture, or a certain fraction thereof, is measured, and this enzyme activity is a measure of the quantity of the component to be estimated. A larger number of variations on this immunological test method, and use of different techniques, are known. A possible way of performing the test for the demonstration of an antigen, for example HCG for the demonstration of pregnancy, consists of incubating a urine sample with a certain quantity of a coupling-product of HCG and an enzyme, and a certain quantity of an antibody against HCG, which has been rendered insoluble or may afterwards be rendered insoluble. A competitive reaction takes place between antibody and the HCG to be demonstrated on the one hand and between antibody and HCG coupled to enzyme on the other hand. After the immunochemical reaction has ceased, the enzyme activity in the solid or liquid phase of the reaction product finally obtained is measured, the said activity being a measure of the HCG to be estimated. Use is preferably made of a peroxidase as enzyme-label in this enzyme immunotest, one of the reasons being that the activity of such an enzyme may be readily determined by colourimetric means, which means that the estimation in qualitative determinations may occur visually.
Test combinations which are marketed for the performance of enzyme immunotests therefore contain as an essential constituent a certain amount of an immunological component coupled to a peroxidase, whereby the reagents are preferably provided in a lyophilized state. As noted above, the major disadvantage of peroxidases is however that their qualities in low concentrations, either in dissolved or suspended state, are not kept over any substantial period of time. The peroxidase activity may also decrease during, or as a result of, freeze-drying. For these reasons, the attractiveness of the use of peroxidases is nullified.